The two leaders said there was "much more to do" to stop the brutal killing of civilians in the Middle East state.

Earlier this week President Obama warned that any movement of Syria's stockpile of chemical weapons would be a "red line" which would have "enormous consequences".

Mr Cameron also discussed the situation with French President Francois Hollande and the two European nations agreed to "work more closely to identify how they could bolster the opposition and help a potential transitional Syrian government after the inevitable fall of Assad".

Downing Street said Mr Cameron and President Obama "both agreed that the use - or threat - of chemical weapons was completely unacceptable and would force them to revisit their approach so far".

A spokeswoman added: "The Prime Minister restated the risk to the wider region posed by the fighting and the fact that regional and international cooperation was vital. He reinforced the need to work in close cooperation with Turkey, Qatar, Saudi Arabia and others on the issue."

The leaders said they hoped a meeting of opposition groups in Cairo would show "real unity of purpose and coherence in working towards transition".

Mr Cameron and Mr Obama agreed that more should be done by the international community to channel humanitarian aid to Syrian refugees through the UN appeal.

Mr Cameron and Mr Hollande discussed the need to maintain international pressure on the Assad regime. They agreed that the refugee situation was "deeply troubling" and Mr Hollande said that the humanitarian crisis would be the focus of France's United Nations Security Council ministerial meeting at the end of the month.